Investment casting is a process for forming articles, often referred to as castings, from metals and alloys. Molten metals or alloys are poured into shells having an internal cavity shaped in the form of a desired article and allowed to solidify into castings. The shell is formed by serially applying layers of materials, such as ceramics, on wax or polymeric "patterns" formed in the shape of the desired article. The innermost layer of the shell, i.e., the first layer deposited on the pattern, is referred to as the "facecoat". The facecoat contacts the molten metal during the casting process. Subsequent layers that are deposited over the facecoat are referred to as "backup layers". Final "seal dip" layers are applied over the underlying backup layers.
Investment casting brittle metals and alloys, particularly reactive materials such as the intermetallic alloys, has proved difficult. Intermetallic alloys are mixtures of two or more metals that form crystal structures different from any of the constituent metals. The intermetallic alloys generally undergo a ductile-to-brittle transformation during "cool down," the period during which the metal or alloy cools in the shell after pouring, or after the metal solidifies inside the shell. Conventional shells historically used in the investment casting process can cause intermetallic castings to crack. Intermetallic alloys also may react with the facecoat during the casting process, particularly at the elevated temperatures at which such metals and alloys are cast. This reaction may cause surface defects, and render the casting useless.
However, intermetallic alloys also have physical properties that make them useful for a number of applications. NiAl intermetallic alloys, for example, have lower densities, higher thermal conductivities, higher melting temperatures and greater oxidation resistance than the known nickel-based superalloys. See Aimone et al.'s U.S. Pat. No. 5,297,615 (Aimone).
Aimone discloses a method for casting intermetallic alloys using shells having a sacrificial backup layer. The sacrificial layer is formed using ceramic materials stuccoed with what Aimone refers to as nonsinterable materials, such as graphite, plastics or other vaporizable materials. The stucco materials are vaporized to form internal crushable voids in one or more of the backup layers. This forms a sacrificial layer, or perhaps plural such layers, which ostensibly reduces the occurrence of defects in the intermetallic casting. However, the volume of the material forming the sacrificial layer does not itself undergo a volumetric reduction.
Sacrificial layers weaken the entire shell, which may cause the shell to leak after molten metal or alloy is poured into the shell. This obviously affects the quality of the casting and wastes raw material. Furthermore, shells having sacrificial backup layers may not maintain the correct dimensions throughout the casting process, which is referred to as dimensional instability. If the shell cannot maintain the desired dimensions, the casting formed using such shell cannot have the correct dimensions. The dimensional stability of investment casting shells may not present a problem when casting small parts. However, shell dimensional stability becomes more important as the complexity of the casting increases and/or the thickness of the casting decreases.
Commercially viable techniques for investment casting intermetallic alloys and other brittle but useful materials are limited. As a result, the investment casting industry still requires reliable shells and methods for casting brittle, reactive materials.